1. Field of the Invention
The present invention relates to a data transfer apparatus which accepts data transfer requests from a plurality of interfaces, and adjusts the sequence of processing the data transfer requests, and a method of controlling the apparatus, and also to a printing system using such a data transfer apparatus.
2. Description of the Related Art
Conventionally, an apparatus which simultaneously accepts data transfer requests from a plurality of interfaces and processes the requests is disclosed in, for example, Japanese Unexamined Patent Publication JP-A 9-286147 (1997). The apparatus comprises an auxiliary memory which is used for accumulating data transferred from the interfaces. Hereinafter, this disclosed apparatus is referred to as prior art 1.
Japanese Unexamined Patent Publication JP-A 10-228364 (1998) discloses a printer apparatus which sequentially accepts data transfer requests from a plurality of interfaces and processes the requests. Hereinafter, this disclosed apparatus is referred to as prior art 2. The prior art 2 printing apparatus is configured so that, during a period when a data transfer from a host apparatus which issues a data transfer request is accepted (hereinafter, such a host apparatus is referred to as first host apparatus), a data transfer request from another host apparatus cannot be accepted (hereinafter, such a host apparatus is referred to as second host apparatus). In the printer apparatus, therefore, the data transfer request from the second host apparatus is accepted after the data transfer from the first host apparatus is ended. In other words, data transfer requests from host apparatuses are sequentially accepted by the printer apparatus to be processed. As a method of performing the sequential acceptance, two methods, or (1) a prior art 2 method as shown in FIG. 8, and (2) a prior art 2 method as shown in FIG. 10 are disclosed.
In the prior art 2 method as shown in FIG. 8, if the second host apparatus issues a data transfer request during a period when the first host apparatus is performing a data transfer, the printer apparatus transmits NAK information to the second host apparatus to notify that the data transfer from the second host apparatus cannot be accepted. The second host apparatus repeatedly issues the data transfer request until the data transfer of the first host apparatus is ended.
In the prior art 2 method as shown in FIG. 10, if the first host apparatus issues a data transfer request, the printer apparatus transmits BUSY information to the second host apparatus. When the data transfer of the first host apparatus is ended, the printer apparatus transmits READY information to the second host apparatus. If the second host apparatus which receives the READY information has data to be transmitted, the second host apparatus transmits a data transfer request.
The prior art 2 data transfer technique in which data transfer requests are sequentially accepted as described above has a problem in that, when the first host apparatus which has just ended a data transfer issues a next data transfer request immediately after the end of the data transfer, the next data transfer request is often accepted in advance of data transfer requests from other host apparatuses, thereby causing a fear that a certain host apparatus continues to occupy the data transfer.
This problem can be solved by, as in the case of the prior art 1 apparatus, simultaneously accepting data transfer requests and then processing the requests. However, this countermeasure has a problem in that an auxiliary memory of a large capacity is required and hence the production cost is high.